During the manufacture of chemical cellulose pulp in the pulp and paper industry the waste liquor obtained, known as "black liquor", is typically combusted in a soda recovery boiler to recover chemicals and heat. In a conventional recovery boiler the process chemical is recovered by injecting the black liquor into the boiler, so that it dries rapidly and is combusted under reducing conditions, and a melt is produced. The melt primarily contains sodium carbonate, sodium sulphide and sodium sulphate. A number of other compounds are also present, but as far as the pulp manufacture is concerned, the main components are Na.sub.2 CO.sub.3 and Na.sub.2 S. Cooking chemicals are produced from the melt for the manufacture of pulp, and it is often preferable that the sulphidity of the cooking liquors is high. A high sulphidity liquor is preferred at the beginning of the cook, and a low sulphidity liquor in later stages. Thus it is desirable to produce cooking liquors of different sulphidities for different stages of the cook.
The flue gases of a soda recovery boiler contain significant amounts of "fly ash", which is separated in an ash cone and in electrostatic filters prior to the flue gas being removed from the boiler. The main components of the fly ash are sodium sulphate and sodium carbonate. Normally this ash is recirculated to the soda recovery boiler by mixing it with strong black liquor to be introduced into the recovery boiler. This adds to the ash load of the soda recovery boiler and thus also increases the sulphur emissions and intensifies boiler corrosion.
According to the present invention it is possible to prevent the return of ash to the recovery boiler, whereby it is possible at the same time to decrease the sulphur load of the boiler. According to the present invention the chemicals of the ash separated from the flue gases of a boiler are recovered by manufacturing sodium sulphide therefrom, which can be used for producing cooking chemicals, most preferably to increase the sulphidity of cooking liquor produced from the recovery boiler melt.
Often more than 90% of the fly ash of the soda recovery boiler is sodium sulphate. The ash usually contains about 10-20% of the entire sulphur load supplied to the recovery boiler. Sodium sulphate powder is also generated as a byproduct of ClO.sub.2 production, which powder is normally used as a make-up chemical and therefore is dissolved into black liquor before the black liquor is concentrated. This dissolution results a higher sulphur load to the boiler.
Sodium sulphide may be manufactured from sodium sulphate according to the following reaction equation (1): Na.sub.2 SO.sub.4 +C.sub.2 H.sub.4 .fwdarw.&gt;Na.sub.2 S+2 H.sub.2 O+2 CO. Also other hydrocarbons may take part in the reactions.
Ethene, C.sub.2 H.sub.4, may be brought to the pulp mill from outside as additional chemical or it may be produced from the gases at the pulp mill containing methyl mercaptan and/or dimethyl sulphide (DMS). A sulphate mill produces a large volume and variety of sulphurous gases. Gases containing organic sulphur compounds may be generated by pressure heating black liquor, for example according to the process described in U.S. Pat. No. 4,929,307. Methyl mercaptan is oxidized to DMS, which further splits forming hydrogen sulphide and ethene, when the gas is heated, or DMS oxidizes when the pressure of the oxygen is higher, forming sulphur dioxide and ethene.
This invention takes advantage of the reducing gases coming from a black liquor heat treatment unit (U.S. Pat. No. 4,929,307) by using them directly in the reduction of the sodium sulfate based salts (i.e. from the recovery boiler or from ClO.sub.2 generation) without any dissolution into black liquor and without any dead load increase of the boiler. Thus the invention helps to increase boiler capacity and prevent corrosion.
Ethene (or a like hydrocarbon), or gas containing ethene and hydrogen sulphide, is brought into contact with the material containing sodium sulphate, so that sodium sulphide is generated according to the reaction (1) above. By means of this reaction, it is possible to adjust the sulphidity of the cook by adding it to a portion of white liquor, which is generated by causticizing the green liquor produced from the melt of a soda recovery boiler. Thus two different white liquors can be produced, one the original white liquor, the other white liquor enriched with sodium sulphide. The liquor high in sulphur can be used at the beginning of the cook, and the liquor having a lower sulphur content at later stages. Alternatively, it is possible to dose the generated sodium sulphide as a separate chemical directly to the digester, preferably at the beginning of the cook.
By way of the present invention, it is possible to affect positively the sulphidity of the pulp digesting process. The present invention also has the advantage that a portion of the sulphur, the fly ash, does not recirculate to the soda recovery boiler, so that the sulphur load of the recovery boiler diminishes and thus also the sulphur emissions and corrosion are correspondingly decreased.
It is the primary object of the present invention to reduce sulphur emissions from, and corrosion of, a soda recovery boiler, while at the same time facilitating the production of high sulfidity, and different sulfidity, sulfate pulp cooking liquors. This and other objects will become clear from the detailed description and appended claims.